1. Field of the Invention
The present invention relates to a detection device and an inkjet recording device, and more specifically to a detection device and a high-speed inkjet recording device that includes the detection device capable of detecting ink-ejection condition of nozzles in precise manner without requiring halt of printing operations.
2. Related Art
Line-scan inkjet printers are a type of high-speed inkjet printer capable of printing on a continuous recording sheet at high speed, and include an elongated inkjet recording head formed with rows of nozzles for ejecting ink droplets. The head is arranged in confrontation with the surface of the recording sheet across the entire width of the recording sheet. The head selectively ejects ink droplets from the nozzles based on a recording signal and impinges the droplets on desired positions across the width of the recording sheet. At the same time, the recording sheet is transported rapidly in its lengthwise direction, which serves as a main scanning operation, so that images can be recorded at any place on the recording sheet.
Various types of line-scan inkjet printers have been proposed, such as printers that use a continuous inkjet type recording head and printers that use a drop-on-demand type recording head. Although drop-on-demand type line-scan inkjet printers have a slower printing speed than do continuous inkjet type line-scan inkjet printers, they have an extremely simple ink system and so are well suited for a general-purpose high-speed printer.
A drop-on-demand type inkjet recording head disclosed in Japanese Patent-Application Publication No. 2001-47622 is formed with a plurality of nozzles each in fluid communication with an ink chamber and ejects ink droplets through the nozzles by applying driving voltages to energy generation elements, such as piezoelectric elements or heat elements.
In this type of recording head having a plurality of nozzles, when ink ejection condition of one of the nozzles becomes poor, then overall printing quality will be greatly degraded due to undesirable while line appearing throughout printed pages, uneven color density, or the like. For example, a nozzle becomes unable to eject ink droplets when the nozzle clogs up or when air bubbles reside in the nozzle. Also, ejected ink droplets are misdirected when the nozzle partially clogs or when a nozzle surface of the head vicinity of the nozzle is unevenly wet with ink.
In order to prevent such ejection failure, there has been proposed to prevent ink from clinging on a nozzle surface by using a water-repellent recording head or to periodically perform purging operations or wiping operations. However, it has been difficult to completely remove causes of ejection failure.
In view of foregoing, there has also been proposed a detection device that monitors ink ejection condition of each nozzle to detect a defective nozzle. For example, Japanese Patent-Application Publication No. 2001-212970 discloses a detection device that detects ink ejection condition for use in a serial printer. The detection device moves a recording head to a predetermined home position and detects ink ejection condition of each nozzle based on ink droplets ejected from the recording head at the home position. Theoretically, it is possible to use the detection device in a line scan printer.
Japanese Patent-Application Publication No. 2002-103627 discloses a different type of detection device for use in a line scan printer. This detection device utilizes minute ink droplets, such as ink mist generated when abnormal ink ejection occurs. That is, even if a nozzle has become defective, the nozzle usually does not become totally incapable of ink ejection at once, and even defective nozzle can eject ink droplets for a while, albeit in defective manner, causing ink splash or misdirected ink droplets. When such a minute ink droplet impinges on a deflection electrode provided in confrontation with a nozzle row, then an air current is generated in the deflection electrode, based on which poor ink ejection condition can be detected.
However, because the detection device disclosed in Japanese Patent-Application Publication No. 2001-212970 moves the recording head to the home position for detecting the ejection condition, it is necessary to stop printing operations. This decreases throughput of printing. Also, it is difficult to precisely stop and restart scanning movement of the recording head during printing operations in a high-speed line scan printer, the printing operation should not be stopped in a middle of printing. Accordingly, using the detection device disclosed in Japanese Patent-Application Publication No. 2001-212970 in a high-speed line scan printer is not practical.
On the other hand, ink ejection condition can be detected without stopping printing operations when the detection device of Japanese Patent-Application Publication No. 2002-103627 is used. However, if a nozzle becomes incapable of ejecting ink all of a sudden, before causing any ink splash or the like, then the detection device cannot detect defectiveness of the nozzle. Also, if ink mist bounces off a sheet surface and clings on the electrode, then the detection device may erroneously detect a normal nozzle as a defective nozzle.